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Two NASA astronauts installed equipment outside the International Space Station that could eventually lead to a satellite-refuelling station run by robots in space.

The work was carried out during the last spacewalk that will ever be part of a space shuttle mission.

Flight engineers Mike Fossum and Ron Garan completed the 6½-hour spacewalk Tuesday to carry out some minor repairs and a couple of major tasks.

"To see that we've really done it, it's just awe-inspiring," Fossum said of the completed space station and the 160 spacewalks that took place during the 12½-year life of the orbiting outpost. "Ron and I are honoured to be a part of it, to help close out one of the final chapters."

Their spacewalk is considered part of the Atlantis space shuttle mission — NASA's last space shuttle flight ever — but they themselves arrived on a Soyuz spacecraft earlier in the year.

NASA astronaut Mike Fossum prepares for the only spacewalk planned during the Atlantis shuttle's stay at the space station. (NASA)

The two are part of Expedition 28, a long-term mission at the space station set to end in September.

The four astronauts who arrived aboard Atlantis remained inside while Fossum and Garan retrieved an ammonia coolant pump that failed a year ago and had been sitting in temporary storage outside the space station. Loss of the pump left the station with only half its cooling power.

The pump will be taken back to Earth on Atlantis, which is why the spacewalk is considered part of Atlantis's mission.

The pump will be analyzed by engineers to figure out why it failed, and then it will be repaired for use as a spare.

Fossum and Garan also installed the Robot Refuelling Mission experiment, a box of tools and satellite components that is expected to allow the Canadian robot Dextre to test technologies to refuel and repair satellites in space.

In particular, NASA wants to see whether the tools will allow a remotely-controlled robot to handle parts that were originally designed for humans to manipulate.

The robotic workbench — which the astronauts attached to a shelf on Dextre's base — consists of a one metre box holding four customized tools, including a wire cutter and a safety cap removal device, as well as an assortment of knobs, caps, valves and a half gallon of ethanol.

The designers of the experiment, based at Goddard Space Flight Center in Greenbelt, Md., envision robots one day using these methods to fill the fuel tanks of satellites orbiting as high as 35,886 kilometres. That would keep the spacecraft operating longer, instead of becoming expensive pieces of space junk.

Atlantis's crew and the rest of the space station crew spent the day continuing to unload the Raffaello multipurpose logistics module carried up by Atlantis.

The four-tonne container held a year's worth of supplies including food, clothing and spare parts.

It is planned that the space station, 400 kilometres from Earth, will continue to operate until at least 2020.

NASA is turning to private enterprise in the post-shuttle period, so it can meet the White House goal of sending astronauts to an asteroid by 2025 and Mars the decade after that.

The 13-day flight by Atlantis is the last for the 30-year shuttle program. Atlantis is due to return July 21 to Kennedy, where it will go on display at a tourist centre.